The present invention relates to an apparatus for storing information on a recording medium by employing a beam of energy such as a laser beam and, more particularly, to an optical type information read/write apparatus for defining sectors for sequentially storing information units on an optical-disk by using a laser beam and retrieving the information units at a sector unit.
An information record/reproduction (read/write) apparatus which employs a recording medium for storing a relatively large capacity of information on a magnetic disk is heretofore known. According to this magnetic information read/write apparatus, in order to enable retrieval of desired units of information stored on the disk at a high speed, coded information is divided into a plurality of recording units each having a predetermined number of bits, i.e., sectors, and is read/written on a magnetic disk per sector unit. The information is organized in an orderly fashion on such a system by tracks and sectors. Each sector includes, in addition to the information unit written by itself, address data indicating an address on the disk of the sector, header data indicating the starting point of the address data and synchronizing signal data. Accordingly, when a sector address is designated, desired one of information units can be retrieved and read fairly readily at a high speed.
However, recording media for storing a further large capacity of information in a high density format has recently been required due to the progress of modern information processing techniques. This requirement has not been sufficiently satisfied by the conventional magnetic disk system.
An optical-disk can be listed as one of the recording media which can satisfy the above-described requirement. A so-called optical-disk system which performs the read/write operations by using an optical-disk for storing a large capacity of information in a high density format has been noted. According to an ordinary optical-disk system, a write laser beam is converged to a spot having a predetermined diameter such as, for example, approx. 1 .mu.m, and this spot-shaped laser beam is emitted to form a pit having approx. 0.6 .mu.m to 1 .mu.m in width and approx. 1 .mu.m to 2 .mu.m in length on the optical-disk, thereby recording the information. The information thus stored on the optical-disk is read by emitting a read laser beam smaller than the energy power of the write laser beam to the optical-disk.
When information is read and written in accordance with a sectoring technique for applying the optical-disk system thus constructed to the magnetic disk system, there occurs a problem such that, when a desired sector is retrieved or accessed at the reading time, the access error occurrence rate is undesirably increased. The reasons for the above problem will now be described. Since the magnetic disk has relatively large unit information size (e.g., approx. 5 .mu.m to 10 .mu.m of unit information length in a track direction and approx. 100 .mu.m to 200 .mu.m of unit information width in the radial direction of the disk), the access error occurrence rate caused by the magnetic disk itself remains approx. 10.sup.-9. Thus, even with the information stored on the disk in accordance with the above-described sectoring technique, the access error occurrence rate can be maintained actually within an allowable range. On the other hand, the size of the pit formed on the disk is extremely small in the optical-disk system as described above. Accordingly, an ultrafine or otherwise insignificant defect such as extremely small pieces of dust adhering to the optical-disk and/or ultrafine scratches produced on the optical-disk might cause a definite access error. As a consequence, the access error occurrence rate of the disk itself in the optical-disk system is increased to approx. 10.sup.-4 to 10.sup.-5. It is impossible with such large error occurrence rate to perform the effective retrieval or access of one desired information unit from a number of information units stored on the optical-disk in accordance with the sectoring technique.
It was reported, for example, in the 1981 meeting of the Institute of Electronics and Communication Engineering of Japan, 1166, Apr. 1-3, 1981, by the inventors of the present invention in a paper entitled "Discussion of error-correction system adapted for optical-disk memory", that there was a possibility of occurring not only "random defects" of micrometer order in size but so-called "burst defects" of larger than 100 .mu.m in size on the optical-disk manufactured in an ordinary state. Accordingly, the elimination of this problem so as to perform the efficient access of the information of the optical-disk system in accordance with the above-described sectoring system has been eagerly awaited, especially since the address data error was occurred in the sector due to these defects and the access error rate of the optical-disk system was considerably higher than the magnetic disk system.
Also according to the optical-disk system, the optical-disk was rotatably controlled, so as to improve the common storing capacity, in accordance with the constant linear velocity (CLV) technique. The rotation of the optical-disk is controlled in this case such that the relative velocity between a laser head and the disk is maintained substantially constant. In this case, the sectors formed on the disk have equal circular-arc-shaped sector lengths. As a result, the number of sectors contained in each track of the optical-disk depend upon the different tracks, and since the starting and ending positions of the sectors contained in the tracks are different from each other, they become irregular. This caused further undesirable deterioration in the access error occurrence rate in the optical-disk system. When such problems are eliminated in the hardware and in a system configuration of the optical-disk system, the structure of the optical-disk system becomes undesirably complicated.